Dreana and Elara Dorne are married and planning to have a family some day. I think those two have had more on screen kisses than any of my other characters and companions so far. Though Rojavan and Risha had more blank screens indicating intimate moments.

If your married your companions email you some gifts, which I always give to a different companion since married ones already maxed affection, but otherwise no advantage that I've seen yet. So far no other nPCs, not even other companions, seem to know about the marriage.

I think its funny that when Silvary arrived on Hoth, the ice planet, Malavi Quinn (medi) refuses to leave the ship and Vette asks to be left onboard as well. She pretends to have a cold. Lieutenant Pierce and Jaesa Willsaam are both willing to go. Of course you can actually summon any of them if you want. I'm working on getting Jaesa to 1000 kills so I wanted her on the planet anyway.